Au Gratin Potatoes

Ingredients
4 medium potatoes, sliced into 1/4 inch slices
1 small  onion, sliced into rings
3 tablespoons butter
3 tablespoons all-purpose flour
1/2 teaspoon salt
2 cups milk
1 1/2 cups shredded Cheddar cheese

Preparation:

1. Preheat oven to 400 degrees F.
   
2. Butter a casserole dish.
   
3. Layer 1/2 of the potatoes into bottom of the prepared casserole dish. Season with salt and
   pepper to taste.
   
4. Top with the onion slices, and add the remaining potatoes. Season with salt and pepper to taste.
   
5. In a medium-size saucepan, melt butter over medium heat. Mix in the flour and salt, and stir
   constantly with a whisk for one minute.
   
6. Whisk in milk.
   
7. Cook until mixture has thickened.
   
8. Stir in cheese and continue stirring until melted.
   
9. Pour mixture over the potatoes, and cover the dish with foil.
   
10. Bake about an hour until potatoes are soft enough to eat.